Torsional sensor for a power-assisted steering device

ABSTRACT

A steering mechanism has an input shaft 2, a steering valve 1, an output shaft 3, and a torsionally elastic element 5. The input shaft 2 has a first longitudinal axis about which the input shaft 2 is rotatable, the input shaft 2 being rotatable about the first longitudinal axis to effect the steering of a vehicle. An end of the input shaft 2 is located in the steering valve 1. The output shaft 3 has a second longitudinal axis coaxial with the first longitudinal axis and a first end adjacent the end of the input shaft 2. The torsionally elastic element 5 flexibly connects the first end of the output shaft 3 and the end of the input shaft 2 such that the input 2 and output 3 shafts are able to rotate relative to each other about the first and the second longitudinal axes, respectively, during vehicle steering. The torsionally elastic element 5 includes a plurality of separately arranged strips 8, 9, 10 &amp; 11 located in the steering valve 1. The plurality of separately arranged strips 8, 9, 10 &amp; 11 have longitudinal axes and are adapted to twist during vehicle steering.

The invention relates to a steering valve with an input shaft, an outputshaft and a torsionally elastic element connected at one end to theinput shaft and at the other end to the output shaft.

Steering valves, particularly so-called rotary disk type valves, areused by preference in power-assisted steering systems. Here, forexample, hydraulic control means are used to shift hydraulic steeringbores relative to each other as a function of the relative torsionbetween input and output shaft, which in turn depends on the springcharacteristic of the torsionally elastic element, such that hydraulicfluid flow occurs. Other control types are conceivable.

In order to be able to use such systems in a defined controlled manner,a wide variety of measures are taken to detect signals by means ofsensors. However, in practice, no direct measured values are generatedbut only indirect functional variables based on which the respectivedesired value can be determined. It is desirable, for example, togenerate steering torque dependent electrical signals and use these forsteering control. For this purpose, the steering torque can be convertedinto a measurable relative motion of two components by means of atorsion element of a steering device of the generic class or the straincan be measured directly on the surface of the torsion element. Variousinexpensive methods for measuring strain with great accuracy are known.However, their use for measuring steering torques is problematic becausethe steering shaft (input shaft) is not only subject to torsion but alsoto bending and in a conventional torsion element with a roundcross-section, the surface tensions due to bending moments compared tothe surface tensions due to torsional moments are too small.

Mechanical decoupling of the two stress types, for example by means of amultiple bearing arrangement of the steering shaft or a guide bush, notonly substantially increases the costs to build the steering device but,due to unavoidable bearing friction, can also lead to unacceptablehysteresis in the steering torque signal.

DE 27 34 182 A1 describes a device for measuring steering torques andsteering angles in vehicles. A steering shaft and a dynamo hub that canbe connected thereto are arranged in the steering wheel area to measuresteering torques. An additional measuring element comprises an upper anda lower ring which are connected to each other by spokes that arearranged in fish-trap fashion and provided with strain gauges. Such anadditional measuring element entails a redesign of conventional steeringvalves and a substantial increase in size. A torque sensor is disclosedin EP 0575634 A1.

Thus, it is the object of the present invention to further develop asteering valve of the generic class so that on the one hand steeringtorques result in easily measurable surface strains on a component andon the other hand bending stresses of the steering shaft (input shaft)do not appreciably influence the measuring result.

To solve this technical problem, the invention proposes that thetorsionally elastic element be made of a plurality of separatelyarranged strips.

The solution according to the invention makes it possible that steeringmoments essentially result in a bending stress of the strips with easilymeasurable surface strains, whereas bending moments on the steeringshaft (input shaft) essentially cause only tensile or compressivestresses of the strips with surface strains that are very minor bynature.

The solution according to the invention now makes it possible toevaluate the mechanical strain of at least two of the strips forming atorsion element by means of measurements and thus precisely to measure,for example, the steering torque.

The invention proposes to arrange the strips such that they extend inaxial direction of the input and output shaft, respectively, and arespaced apart from each other. It is particularly advantageous if thestrip arrangement is ring-shaped.

The described advantageous embodiment of the invention thus replaces aconventional torsion element by strips that are arranged in the shape ofa ring; that is, the bar-shaped strip elements, which according to oneproposal of the invention are advantageously rectangular or trapezoidalin cross section, are arranged in such a way that they extend along thesurface of an imaginary cylinder, parallel and spaced apart from eachother, and are fixed at one end to the input shaft and at the other endto the output shaft. The imaginary cylinder, for its part, is concentricto the longitudinal center axis of the unit formed by the input and theoutput shaft. If the input shaft is twisted with respect to the outputshaft, the strip elements are uniformly stressed such that, for example,the signal representing the torque can be measured directly. The stripsmay be interconnected at least at one end by a ring element. Such ringelement may in turn be connected to one of the two shafts.

With particular advantage, the invention proposes that each separatestrip have a defined deformability so that signal measurement issignificantly simplified.

To measure the signals, the invention advantageously proposes to arrangestrain sensors on the strips which advantageously are strain gauges orso-called SAW sensors, that is, surface acoustic wave devices.Advantageously, the strain gauges are disposed so that they can beevaluated in a compensating manner.

Finally, the invention proposes the wireless transfer of the measureddata. Alternatively, it is of course possible to use vires, for examplespirally wound ribbon cables, such that relative twists cannotnegatively affect the measured data transfer, or wireless transfermethods, for example, inductive methods may be used. Compensatingconnectability means, for example, that the sensors of opposite stripsare used for measurements. This has the particular advantage thatbending moments can be compensated. It is particularly advantageous iffour strips are arranged at 90° intervals, respectively. The straingauges can easily be mounted on the readily accessible side areas of thestrips so that expensive sensors, for example SAW sensors, may be usedcost effectively even in series production.

Finally, the invention proposes to provide overload protection toprevent destruction of the design according to the invention. Forexample, overload protection may consist of the fact that the inputshaft and the output shaft run into each other over a given axiallength, whereby the respective outer shaft has an enlarged rectangularlocation opening in which a rectangular end with shorter edges of therespective other shaft is inserted. Thus, starting with a specific witha specific torsion angle, the two shafts are in contact and the torquetransmitting strip arrangement cannot be further stressed.

The invention provide a simple and economical steering valve making itpossible to pick up steering torque signals and generally pick upelectrical signals for control purposes.

Additional advantages and characteristics of the invention are describedbelow by means of the figures.

FIG. 1 is a schematic representation of a steering value.

FIG. 2 is a schematic representation of an exemplary embodiment of atorsion element.

FIG. 3 is a section along Line III--III according to FIG. 2.

FIG. 4 is a schematic representation of stressed strip elements.

FIG. 5 is a perspective view of a stressed torsionally elastic elementwherein the stress zones are screened.

FIG. 1 shows the schematic structure of a power-assisted steering devicea steering value 1, comprising an input shaft 2 and an output shaft 3mounted in a housing with the output shaft 3 engaged, for example, witha rack 6 of a steering gear. At the location where the input shaft andthe output shaft run into each other, the input shaft 2 has arectangular segment on its end face which projects into a rectangularrecess in the output shaft 3. The rectangular recess of the output shaft3 is enlarged with respect to the rectangular segment of the inputshaft, such that the two shafts 2, 3 can be twisted with respect to eachother by a predetermined angle until they come into contact. Within thistwistability range, the two shafts are prestressed by a torsionallyelastic element. The device described above makes it possible togenerate measured values representing the steering torque by measuringthe relative twist between input shaft 2 and output shaft 3. To permitthe generation of measured values that represent the steering torque bymeasuring surface strain, a torque strip arrangement 5 is used.

FIGS. 2 and 3 show an exemplary embodiment for a torque striparrangement 5. In the embodiment shown, input shaft 2 is provided with adisk-shaped flange 7 on its end face with which strips 8, 9, 10 and 11form a single unit. Such strips are disposed parallel to each otheralong the surface of an imaginary cylinder 12, coaxially to the unitformed by input shaft 2 and output shaft 3, and are provided at theirother end with an additional integral end flange to which output shaft 3is fixed by weld seam 13. Input shaft 2 can be continued centrally orconcentrically in any desired thickness and project into output shaft 3and/or vice versa, to form an overload protection in the above describedmanner.

FIG. 4 schematically represents the nature of the strain of strips 14,15 when a force is applied to a torque strip arrangement in thedirection of the arrows. The input shaft is twisted clockwise againstthe output shaft. Strips 14 and 15 equipped with sensors 22, 23, 24 and25 deform in the manner shown. Electrical or electronic analysis of thestrain sensors, either strain gauges or SAW devices, makes it possibleto measure a signal representing the torque. The fact that oppositestrips are used for the measurement permits full compensation of, forexample, bending moments.

As shown in plan view in FIG. 3, the strain gauges can easily be mountedon the side edges of the strips if four strips are employed becauseaccess is completely free. Thus, this arrangement is particularlysuitable for industrial series production.

FIG. 5 finally shows an exemplary embodiment of a torsion striparrangement 5 where square bars 18, 19, 20 and 21 are inserted betweentwo rings 16, 17. The upper ring is twisted counter-clockwise withrespect to the lower ring such that the indicated strains result. Thescreened areas clearly show the strain centers. With the use ofappropriate strain sensors, the desired measuring signals can now beeasily derived.

LIST OF REFERENCE NUMBERS

1 Steering device

2 input shaft

3 output shaft

5 torque strip arrangemnent

6 rack

7 end flange

8 strip

9 strip

10 strip

11 strip

12 imaginary cylinder

13 weld seam

14 strip

15 strip

16 ring

17 ring

18 strip

19 strip

20 strip

21 strip

22 sensors

23 sensors

24 sensors

25 sensors

φ_(rel) angle

I claim:
 1. A steering mechanism comprising:an input shaft having afirst longitudinal axis about which said input shaft is rotatable, saidinput shaft being rotatable about said first longitudinal axis to effectthe steering of a vehicle; a steering valve in which an end of saidinput shaft is located; an output shaft having a second longitudinalaxis coaxial with said first longitudinal axis and having a first endadjacent said end of said input shaft; and a torsionally elastic elementwhich flexibly connects said first end of said output shaft and said endof said input shaft such that said input and output shafts are able torotate relative to each other about said first and said secondlongitudinal axes, respectively, during vehicle steering; saidtorsionally elastic element comprising a plurality of separatelyarranged torsionally elastic strips located in said steering valve, saidplurality of separately arranged strips having longitudinal axes andbeing adapted to twist during vehicle steering.
 2. The steeringmechanism of claim 1 wherein said longitudinal axes of said stripsextend in a direction parallel to said longitudinal axes of said inputand said output shafts.
 3. The steering mechanism of claim 2 whereinsaid strips are joined by at least one ring element.
 4. The steeringmechanism of claim 1 wherein said strips are circumferentially spacedabout said longitudinal axes of said input and said output shafts. 5.The steering mechanism of claim 1 wherein said strips have a rectangularcross section taken through a plane perpendicular to said longitudinalaxes of said strips.
 6. The steering mechanism of claim 1 wherein saidstrips have a trapezoidal cross section taken through a planeperpendicular to said longitudinal axes of said strips.
 7. The steeringmechanism of claim 1 wherein a second end of said output shaft isengaged with a rack of a steering gear.
 8. The steering mechanism ofclaim 1 wherein said strips are equipped with sensors.
 9. The steeringmechanism of claim 8 wherein said sensors are strain gauges.
 10. Thesteering mechanism of claim 8 wherein said sensors are SAW sensors. 11.The steering mechanism of claim 8 wherein said sensors comprisecompensating sensors.